It has been shown that TV sensing devices are capable of sensing incumbent digital television (DTV) signals down to a level of −114 dBm using, for example, a white space spectrum sensor as described in United States patent application 20110043710A1 published Feb. 24, 2011.
FCC 47 CFR Part-15 and FCC 10-174 specify that TV Band sensing devices are expected to sense incumbent DTV signals down to a level of −114 dBm in the presence of adjacent channel DTV signals that can be as strong as −28 dBm. Known sensing devices are incapable of sensing the presence of a DTV signal at a level of −114 dBm in the presence of strong adjacent channel interference in the order of −28 dBm.
The channel signal-to-noise ratio (SNR) encountered by a TV Band sensing device can be as low as −14 dB even for a TV Band sensing device with an analog front-end noise figure as low as 6 dB. One technique that can be employed to sense the presence of an incumbent DTV signal when the SNR is very low involves sensing only for the DTV Pilot tone. The DTV Pilot tone is a pure tone that has a designated power level that is 11.62 dB below the transmitted total DTV signal power.
However, the presence of a strong −28 dBm adjacent-channel interference poses a significant challenge for sensing the presence of the DTV pilot tone. Moreover, the presence of adjacent-channel interference generated by lower frequency (left adjacent) channel makes pilot-sensing even more difficult because the strong out-of-band emissions from the left-adjacent channel masks the pilot position side of the 6 MHz TV channel bandwidth.
It can be shown by computation that the Pilot Carrier-to-Noise Ratio (CNR) in a 1 Hz bandwidth is about 6.37 dB, assuming that a left adjacent channel of −28 dBm is present with a sensed channel DTV signal level of −114 dBm. This computation assumes that the left adjacent-channel DTV signal conforms precisely to the FCC DTV emissions mask.
For any given CNR ratio, there is a finite probability of detecting an incumbent signal. There is also a finite probability of false alarm, i.e., an incumbent signal is “declared” even though it is not actually present. To avoid interference with an incumbent DTV signal, it is important to keep the probability of detection high. However, in order to permit secondary user access to unoccupied TV channels, it is important to keep the probability of false alarm low.
The FCC has set a probability of detection of over 90%, with a probability of false alarm not exceeding 10% as an acceptable sensing performance for a TV Band device in sensing for incumbent DTV signals. The FCC has further required that each DTV channel be sensed over a minimum interval of 30 seconds before declaring the presence or absence of an incumbent signal.
Detecting the presence of the pilot at a CNR of 6.37 dB (over 1 Hz) to meet the above detection and false alarm specifications, and accomplishing that in less than 30 seconds is a formidable challenge.
There therefore exists a need for a method and apparatus for detecting the presence of a DTV pilot tone in a high noise environment.